Analog-to-digital (A/D) converters are electrical circuit devices that convert continuous signals, such as voltages or currents, from the analog domain to the digital domain where the signals are represented by numbers. A variety of A/D converter types exist, including flash A/Ds, sub-ranging A/Ds, successive approximation A/Ds, and integrating A/Ds. Another type is known as a sigma delta or delta sigma (e.g., Δ-Σ) A/D converter that includes a delta sigma modulator operating as a type of noise shaping encoder, typically with a 1-3 bit quantized digital output. Delta sigma or sigma delta modulators are often used in mixed signal integrated A/D converters, because of their insensitivity to CMOS process linearity and matching problems when compared to other A/D converter types. These features make delta sigma based mixed signal solutions very attractive for a number of applications, such as audio, receiver channels of communication devices (wireless in particular), sensor interface circuits, and measurement systems.
Delta sigma converters are operated at a significantly higher sampling rate than the bandwidth of the analog input signal, a technique referred to as oversampling, wherein the analog input signal is sampled at a very high sampling rate in order to perform a noise shaping function. The oversampling is commonly performed at a multiple of the Nyquist rate (FN) for a given input signal frequency content (e.g., sampling frequency FS is often 10 to 1000 times FN), wherein quantization noise power is spread over a bandwidth equal to the sampling frequency, thereby reducing the noise density in the band of interest. A noise shaping or loop filter, typically a lowpass filter (e.g., integrator), is commonly provided in the forward signal path of the delta sigma modulator to push some of the quantization noise into the higher frequency spectrum beyond the band of interest. Digital filtering is performed on the oversampled digital output to achieve a high resolution, and decimation is employed to reduce the effective sampling rate back to the “Nyquist” rate.
In addition to traditional lowpass loop filters, delta sigma modulators often include bandpass loop filters where a signal of interest lies within a certain frequency band. For example, in wireless communications devices, bandpass delta sigma A/D converters are employed to ascertain a signal of interest in narrow or wide bands (e.g., Global Systems for Mobile communications (GSM), Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), etc.) using a bandpass loop filter to reduce noise outside the particular band of interest. In this regard, GSM applications typically involve channel bandwidths of 200 kHz, the Bluetooth CDMA standard corresponds to the 600 kHz and 1.0 MHz bandwidths, WCDMA and Bluetooth standards correspond to 2.0 and 3.0 MHz bandwidths, with video standards using even wider bands. Conventional delta sigma modulation based bandpass A/D converters implement the noise shaping transfer function for the loop filter using analog components, including inductors and other highly nonlinear circuit elements. These circuits, once constructed, are optimized for a fixed bandwidth and a fixed sampling frequency FS, typically about 4 times the center frequency of the band of interest (e.g., the center frequency is about ¼ of FS).
However, in certain receiver applications, different channels are located at different center frequencies. For example, switching from one channel to another in a GSM receiver requires changing the center frequency of the A/D converter loop filter. In conventional designs, this has been accomplished using a variable sampling clock frequency to address different frequency bands. Variable clocks, however, require complex phase locked loop (PLL) circuitry, thus increasing the circuit area occupied by the converter and the cost. In other situations, it is desirable to create a bandpass delta sigma modulator that may be used in both narrow band and wider band applications. For instance, the same bandpass A/D converter may need to convert signals in one or more narrow GSM bands, as well as in wider bands for CDMA and WCDMA applications. Conventional designs do not allow this flexible operation, since the analog bandpass loop filter poles and zeros (e.g., and hence the passband width) are fixed, making it difficult to address multiple application standards. Furthermore, analog bandpass filters for such delta sigma modulator applications include highly non-linear components (e.g., inductors, etc.), which are difficult to implement in CMOS fabrication processes and integrators with stability shortcomings, high signal swings, high area usage, and high power consumption. Accordingly, there is a need for improved bandpass delta sigma modulators and A/D converters to allow operation with different bandwidths and center frequencies.